Tantalum, and niobium are generally extracted from their ores in the form of powders. For example, tantalum is generally produced by reducing potassium fluorotantalate (K.sub.2 TaF.sub.7) by chemical reaction with sodium. This reduction reaction generally produces a salt-encapsulated metal powder which is crushed and washed, with water and acid, to produce tantalum powder.
Tantalum and niobium metals, and their alloys, are then consolidated to form products. The method chosen for consolidation depends upon whether the resulting consolidated product will be pure metal or an alloy, what form or shape is required, and how the material is to be used. Tantalum, niobium, and their alloys are generally used to form wrought products, such as bars, plates, sheets, wire, tubes and rods; preforms, for subsequent thermo-mechanical processing; and near net shapes, for use, in a variety of applications, after machining and finishing.
Tantalum, niobium and their alloys generally have a high affinity for oxygen. Thus the oxygen content of products of niobium, tantalum, or their alloys tends to increase during their formation. The oxygen content of the product affects its mechanical properties and fabricability. Generally, as the oxygen content of the product increases, the product's ductility decreases and the product's strength increases. For many applications utilizing products of tantalum, niobium, or their alloys, a high oxygen content is unsuitable. Therefore, to produce tantalum, niobium, or alloy products suitable for these applications, a low oxygen content must be obtained.
There are several methods which may be utilized to produce formed products of tantalum, niobium or their alloys. For example, in one method the metal is first melted by electron beam or vacuum arc melting, in a vacuum, and then thermo-mechanically processed to form the product. The melting temperature is also referred to as the homologous temperature (T.sub.H) in degrees Kelvin. T.sub.H for tantalum is 3273 degrees K and T.sub.H for niobium is 2745 degrees K. The melting in a vacuum reduces the oxygen content of the metal.
In a second method the metal, in powder form, is first cold isostatically pressed into a tantalum, niobium or alloy preform, such as a bar or rod, and then the preform is resistance sintered at a temperature greater than 0.7 T.sub.H to produce a formed product of tantalum, niobium or their alloys. Generally, for resistance sintering, the ends of the preform are clamped between water cooled copper terminals in a high vacuum chamber and then the preform is heated, to a temperature above 0.7 T.sub.H, by passing an electrical current through the preform. The resistance sintering simultaneously densifies and lowers the oxygen content of the preform.
However, there are many disadvantages in utilizing resistance sintering to densify and remove oxygen. First, resistance sintering may only be utilized to produce products of certain limited shapes, generally bars or rods. For resistance sintering, the crosssection of the preform must be uniform along the path of electrical current in order to prevent localized overheating and hot-shorting. Additionally, the cross section must be small enough so that the oxygen reduction in the center of the preform occurs before the disappearance of the interconnected porosity. For effective oxygen removal, preforms greater than about 1.5 inches in their shortest dimension are not resistance-sintered. Still further the preform must be small enough to prevent sagging associated with creep and hot pressing during unsupported resistance sintering. Thus, the preforms generally do not weigh greater than about 40 lbs.
A third method for producing formed products of tantalum, niobium, or their alloys, is the rotating electrode process. In this process a bar or rod of the metal is heated to a temperature above T.sub.H. The molten metal is converted into powder by centrifugal force. The low oxygen content of the starting rod is maintained in the powder, however the powder particles are relatively spherical and generally coarser than the initial chemically produced powders. These relatively spherical powder particles are not desirable for unidirectional mechanical pressing. Further, the coarseness of the powder particles makes the powder undesirable for cold-isostatic pressing into formed tantalum, niobium or alloy products.